1. Field of the Invention:
The present invention relates to polyurethane compositions and more particularly to a fully cured rocket motor chamber or casing liner for use in the manufacture of a solid propellant motor.
2. Description of the Prior Art:
Liner layers are required in solid propellent motors for several reasons. They are utilized to render a motor casing gas impermeable, to provide a heat insulating film between the casing and the propellant, and also as a material interposed between the chamber wall and the propellant to which the propellant can adhere. Solid propellants based on a polyurethane binder especially those made from hydroxy terminated polybutadiene (HTPB) as one of the carbamate linkage forming compounds have been found to exhibit very poor adherence to liners. This has seriously affected motor system reliability.
Even though the initial bond of the propellant to the liner may appear to be satisfactory, after aging the bond can weaken and reliability of the bond becomes questionable. The most probably cause for change in bond characteristics on aging is the migration of plasticizers and other reactive components from the propellant to the liner during cure of the propellant. This upsets the propellant-binder cure causing the bonding polymer phase to become weaker than the propellant matrix.
HTPB propellants which usually contain as much as 35% plasticizer are generally bonded to liner-formulations which are unplasticized. When plasticizer migrates from the propellant to the liner, several effects can occur. The migration of pure plasticizer from the propellant to the liner layer is not normally considered to be detrimental to adhesion as long as the plasticizer is soluble in both the propellant and substrate liner. However, the migration of impure plasticizer can cause problems.
The propellant may contain a variety of materials such an antioxidants, non-functional prepolymer, and soluble burning rate additives. These materials would ordinarily have very little tendency to migrate from the propellant. The presence of plasticizer in the propellant binder, however, results in a less tight polymer structure such that it is possible for some of said materials to migrate with the plasticizer. The migration of unreacted diisocyanate from the propellant binder into the plasticizer is the most serious cause of poor propellant-liner bond formation. This upsets the binder polymer layer stoichiometry at the interface resulting in a weak boundary layer. Occasionally this phenomena is so severe that the interface material becomes so soft and sticky that the resulting bond strengths are very low.
Various measures have been attempted in order to improve the interfacial bonding. In one approach, a washcoat solution containing a curing agent such as toluene diisocyanate (TDI) in chlorothene was sprayed onto the surface of the cured liner stratum before application of the propellant. However, the liner did not consistently exhibit good bonding to the propellant again causing poor reliability. Poor processability and short storage life were also experienced. Formulation of the liner or propellant with excess curing agent, such as TDI, was attempted but was found to be unsatisfactory due to an adverse affect on the physical properties of the liner or propellant. Partially cured liners were also considered but were not found to be satisfactory due to limited time available to cast the propellant onto the partially cured liner and again adhesion of the propellant to the liner and the liner to the chamber was not always to the desired degree due to the criticality of the time required, often only a few hours, before which casting of the propellant must take place.
Ser. No. 292,196 filed Sept. 25, 1972 by this inventor discloses a recently devised system in which preformed isocyanate dimer of molecular weight less than about 750 is added to a liner composition preferably in combination with monomeric polyisocyanate. This system provided excess stable isocyanate and improved bond and interface characteristics. However, the dimer must be separately preformed and is a crystalline, particulate solid insoluble in the liquid composition and, as such, must be thoroughly dispersed before cure.